Friction materials for brake devices are produced by dispersion a base substance in a resin (such as a phenolic resin or epoxy resin) serving as a binder, incorporating a friction-wear adjusting agent (such as barium sulfate) into the dispersion when so required, and molding the resulting composition with heating and application of pressure.
Chrysotile asbestos fibers have been used as the base substance, whereas with an increase in the temperature of the friction surface, the asbestos-base friction material becomes markedly worn and damaged and is susceptible to a fading phenomenon involving a rapid decrease in the coefficient of friction. Further asbestos fibers are said to have the problem of being carcinogenic.
Accordingly, efforts have been directed to the development of substitutes for asbestos fibers.
Useful as base substances are, for example, alkali metal titanate fibers represented by M.sub.2 Ti.sub.n O.sub.2n+1 wherein M is an alkali metal such as K, Na, Li or Rb, and n is an integer of 2 to 8 (as disclosed, for example, in JP-A-191599/1986). Typical of such fibers is potassium hexatitanate (K.sub.2 Ti.sub.6 O.sub.13).
Alkali metal titanate fibers are synthetic inorganic compounds having a layered crystal structure (when n is, for example, 2, 3 or 4) or tunnel-type crystal structure (when n is 6 or 8), and are materials outstanding in strength, heat resistance and wear resistance and having a suitable hardness as friction materials. Such alkali metal titanates are prepared, for example, by the melting process, i.e., melting a mixture of titanium dioxide and an alkali metal carbonate (about 2 in TiO.sub.2 /M.sub.2 O molar ratio) with heating and cooling the melt to form an alkali metal dititanate (M.sub.2 Ti.sub.2 O.sub.5), followed by a treatment for dissolving out alkali metal ions to adjust the chemical composition and by a heat treatment to alter the crystal structure.
Friction materials prepared with use of alkali metal titanate fibers as the base substance are superior in friction-wear characteristics to the conventional friction material wherein asbestos fibers are used, but it is desired to further improve these materials in anti-fading characteristics and coefficient of friction.
It is proposed to use potassium titanate fibers and alumina-silica fibers in combination in order to give improved anti-fading characteristics and other properties, whereas the alumina-silica fibers are as hard as about 7 in Moh's hardness and undesirable to use since the friction material then obtained has excessive counterpart abrasive properties due to the combination.
Alkaline-earth metal titanates represented by RTiO.sub.3 wherein R is an alkaline-earth metal such as Ca, Ba, Sr or Mg are available as titanium compounds which are excellent in strength, heat resistance, heat-insulating properties, wear resistance and other properties like alkali metal titanates. These titanates are synthetic inorganic compounds having a crystal structure of the perovskite type, and are prepared, for example, by mixing a barium compound powder with acicular particles of hydrous titanium dioxide and heating the mixture (JP-A-88030/1982), by reacting a solution containing a hydrated potassium titanate powder or titanium dioxide hydrate, and bivalent metal ions in a closed container or under a hydrothermal condition (JP-A-113623/1980), by reacting potassium dititanate in the form of crystalline fibers with an aqueous solution of a bivalent metal compound (JP-A-21799/1987), by mixing an inorganic or organic compound of alkaline-earth metal and a flux (e.g., a halide of alkali metal) with an alkali metal salt of titanic acid such as potassium dititanate or potassium hexatitanate serving as a titanium source and heating the mixture (JP-A-164800/1990), or by depositing an alkaline-earth metal carbonate on the surface of a fibroustitanium dioxide compound serving as a starting material and heat-treating the resulting material (JP-A-16917/1991).
Friction materials prepared with use of an alkaline-earth metal titanate powder as the base substance, although possessing a high coefficient of friction, have the drawback of being excessive in counterpart abrasive properties.
In the course of research on improvements in processes for synthesizing the foregoing titanium compounds, we have succeeded in preparing a powder composed of particles of different titanium compounds which particles each comprise crystals of an alkaline-earth metal titanate and crystals of an alkali metal titanate by a relatively simple process using a starting material of adjusted composition, and found that the power is very useful, for example, as the base for friction materials and provides friction materials having a high coefficient of friction but diminished counterpart abrasive properties as the combined characteristics of the different titanium compounds. The present invention has been accomplished based on this finding.